One of the limiting factors for the efficiency of semiconductor components and devices is the recombination of charge carriers at semiconductor surfaces which have surface states that promote the recombination activity. This problem is of importance particularly in solar cells. This is because in this case the recombined charge carriers are no longer available for generating current. In order to reduce recombinations, the semiconductor surface has to be passivated by decreasing the recombination activity of charge carriers by means of surface states.
For the passivation of semiconductor surfaces, at the present time aluminium oxide is used diversely (Al2O3 or AlOx, which very generally is taken to mean any suitable stoichiometric composition of aluminium and oxygen). Aluminium oxide has a high negative surface charge density and, if applied to the semiconductor surface indirectly by way of an intermediate layer or directly, can displace charge carriers from the semiconductor surface into the semiconductor volume. On account of this field-effect-passivating effect, the semiconductor surface is effectively passivated.
Such passivation layers composed of aluminium oxide have the disadvantage, however, that they are not resistant to specific production processes, particularly in the production of solar cells, for example to wet-chemical cleaning or etching processes. Therefore, they have to be protected by means of additional covering layers. This leads to higher production costs and longer production times.
Furthermore, particularly in the case of solar cells, not only is it necessary to take account of the electrical properties of the layer sequences, but it is also necessary for the optical properties of the individual layers to be coordinated with one another. However, if only aluminium oxide is used as material of the passivation layer, then the design of the solar cell is fixed to a specific refractive index (namely approximately 1.6).